Unknown maker, 12/1985. This is visually the most beautiful
tube in mini-9-pin package. The pentode's plate seems to take
more space than the bottle allows, looking like a shortened EL84.
All grids can be seen through the cutouts in the plate. Triode
assembly, piggybacked on its side, is far smaller. The original
ECF 80 was widely used in pushpull audio amplifiers (pentode as
driver, DC-coupled to triode working as paraphase inverter).

Recommended pentode op point is 170V plate and G2; -2.0
control grid bias; 10mA plate and 4mA G2 current. This correponds
to 6.2 mA/V transonductance and gain around 1500. Of course, this
is only a remote indicator of triode mode performance (although
Gm should not change significantly).